docs: def update(cls, resource, name, size, snapshot_profile,
                 background, cmdline=None, kernel=None):
          """ Update this disk. """
          if isinstance(size, tuple):
              prefix, size = size
              if prefix == '+':
                  disk_info = cls.info(resource)
                  current_size = disk_info['size']
                  size = current_size + size
  
          disk_params = cls.disk_param(name, size, snapshot_profile,
                                       cmdline, kernel)
  
          result = cls.call('hosting.disk.update',
                            cls.usable_id(resource),
                            disk_params)
          if background:
              return result
  
          # interactive mode, run a progress bar
          cls.echo('Updating your disk.')
          cls.display_progress(result)